1. Technical Field
The present invention generally relates to a circuit and method for controlling a switch. In particular, the present invention relates to a level shifter for controlling a dynamic, bi-directional high voltage analog switch.
2. Background Art
In the field of ultrasound imaging applications, switches are often utilized in the scanhead to reduce the number of co-axial cables. In order for the switches and the underlying electronic devices to have optimal performance, various factors must be considered. For example, it is important for the gate-to-source voltage of each switch transistor to be constant in order for the switch to have high linearity during conduction. This ensures a minimum of signal distortion that would be detrimental to the image quality.
Heretofore many have attempted to implement various systems for controlling a switch. Two such examples are illustrated in U.S. Pat. No. 4,500,802 to Janutka and U.S. Pat. No. 4,595,847 to Weir. However, no existing switch system is dynamic and allows zero DC bias current independent of switch state (i.e., ON or OFF). Specifically, none of the existing technologies allow the switch to be kept on by the charge on the gates of the switch transistors (i.e., a current is not required to keep ,the switch on, only to turn it on initially) while preventing the voltage in the switch from dissipating. In addition, as electronic devices continue to shrink in size, the need to produce smaller electrical components grows. However, no existing product provides a switch that can reach high voltages (e.g., 200 Volts) while occupying smaller amounts (e.g., 50%) of silicon area. Furthermore, no existing product can provide a switch that can reach 500 Volts in a configuration suited for ultrasound electronics. Still yet, none of the existing technologies provide a system for controlling operation of the switch that requires lower voltage positive power supplies (e.g., 5 Volts and/or 12 Volts). In contrast, the existing control systems typically require voltages commensurate with that passing through the switch (e.g., +100 Volts).
In view of the forgoing, there exists a need for a switching circuit and method whereby: (1) the switch can reach higher voltages (e.g., 500 Volts) while occupying a silicon area suitable for ultrasound applications; and (2) the switch can reach available voltages (e.g., 200 Volts) while occupying less area (e.g. 50%) of silicon. Moreover, there exists a need for a circuit and method that is dynamic and allows zero DC bias current independent of whether the switch is ON or OFF. This will allow the switch to remain ON by the charge on the gates of the switch transistors and will prevent power dissipation from the transistors once charged. A further need exists for a circuit and method that allows the gate-to-source voltage on the switch transistors to remain constant so that any non-linearities of the switch are reduced. A need also exists for a circuit and method that requires lower power supplies for controlling operation of the switch.
The present invention overcomes the drawbacks of existing systems by providing a circuit and method for controlling a dynamic, bi-directional high voltage analog switch. In particular, the circuit and method of the present invention include the switch and a level shifter for controlling the switch. The level shifter generally includes a voltage source, a high negative voltage source, input terminals, transistors, and a diode. Collectively the circuit and method provide, among other things: (1) a dynamic circuit that allows zero DC bias current independent of switch state; (2) a constant gate-to-source voltage; (3) increased switch voltages while reducing silicon surface area occupied; and (4) lower voltage power supplies for driving the circuit.
According to a first aspect of the present invention, a circuit for controlling a switch is provided. The circuit comprises: (1) a level shifter, wherein the level shifter includes: (a) a first level transistor coupled to a switch line; (b) a diode positioned between the first level transistor and the switch line; (c) a second level transistor coupled to a current mirror, wherein the current mirror is coupled to the switch line; and (2) a switch coupled to the switch line, wherein the level shifter controls the switch.
According to a second aspect of the present invention, a circuit for controlling a dynamic, bi-directional high voltage analog switch is provided. The circuit comprises: (1) a level shifter including: (a) a first level transistor coupled to a switch line; (b) a first input terminal coupled to the first level transistor; (c) a diode coupled between the first level transistor and the switch line; (d) a second level transistor coupled to a current mirror, wherein the current mirror is coupled to the switch line; (e) a second input terminal coupled to the second level transistor; and (2) a dynamic, bi-directional high voltage analog switch including: (a) a first switch transistor; (b) a switch input terminal coupled to the first switch transistor; (c) a second switch transistor coupled to the first switch transistor; (d) a switch output terminal coupled to the second switch transistor; and (e) a Zener diode coupled between the first and second switch transistors.
According to a third aspect of the present invention, a method for controlling a bi-directional switch having a Zener diode and a plurality of switch transistors with a level shifter is provided. The method comprises the steps of: (1) setting a first input terminal of the level shifter to approximately 0 Volts, wherein the first input terminal is coupled to a first level transistor; (2) setting a second input terminal of the level shifter to approximately 12 Volts, wherein the second input terminal is coupled to a second level transistor; (3) passing a control signal from a voltage source through the first level transistor and a diode of the level shifter to the bi-directional switch; (4) charging the Zener diode and the switch transistors with. the control signal; (5) passing a switch signal from a switch input to a switch output when the Zener diode exceeds a threshold voltage of the switch transistors; and (6) ceasing the control signal when a gate potential of the switch transistors reaches a predetermined potential.
Therefore, the present invention provides a circuit and method for controlling a dynamic, bi-directional high voltage analog switch. The present invention reduces the problems associated with existing systems set forth above.